Nokia RX-51: Fix invalidating zImage kernel format

Prior starting copy of kernel image to target location, invalidate also
zImage magic header. This ensures that on target location would be image
with valid header only in the case valid header was also in the source
location and copy from source to target finished successfully. Copy is
always skipped when kernel image in source location is invalid.

Add also comment to the code which explain what is the code doing.

Fixes: cc434fcc ("Nokia RX-51: Add support for booting kernel in zImage format")
Signed-off-by: Pali Rohár <pali@kernel.org>